Acute lung injury with its associated damage to the pulmonary epithelium and the resultant fibrosis affects approximately 20/100,000 /year in the US. Pulmonary epithelial cells, growth factors and their cognate receptors play an active role in this recovery. However, which growth factors and receptors participate and what processes they direct remains unclear. My laboratory has identified a membrane bound receptor tyrosine kinase family in the lung, the human epidermal growth factor-like receptor family (HER2, 3 and 4), whose expression is localized to the pulmonary epithelium. Our studies of the HER system and its ligand, Neuregulin-1 (NRG-1), suggest they play an important role in the recovery of pulmonary epithelial cells from injury. This knowledge has led us to hypothesize that: Activation of the HER2/HER3 receptor in pulmonary epithelial cells during or after lung injury directs a fibrotic response via the elaboration of pro-fibrotic cytokine cascades and promotes epithelial cell repair. The goal of this proposal is to test this hypothesis in vivo using transgenic mice with lung specific defects in the NRG-1/HER2/HER3 axis and define mechanisms of receptor activation and downstream signaling in vitro. In this application we propose to: 1) Define the HER2/HER3 receptor complex's role in the regulation of fibrotic cytokine cascades using unique transgenic mouse strains with lung specific expression of a dominant negative HERS receptor, 2) Define mechanisms of HER2/HER3 receptor activation during injury that results in fibrotic cytokine production, and 3) Use a dominant negative STAT3 molecule to understand the HER2/HER3 induced activation of STAT3, and its role in repair of the epithelium. Bleomycin and mechanical injury models will be used in vivo and in vitro to initiate lung injury and determine the effect(s) of HER2/HER3 receptor activation and inactivation on the epithelium's ability to direct fibrosis, inflammation, proliferation, and apoptosis. The identification of growth factors, receptors, and induced signals important in the pulmonary repair process will lay the groundwork for identification of new therapeutic strategies for patients with fibrotic or otherwise damaged lungs.